NASA's biggest asteroid threat: The asteroids NASA can’t see and why they’re the ones that worry scientists most

It’s not the giant, planet-ending asteroids from Hollywood movies that concern NASA the most. And it’s not the tiny space rocks that burn up harmlessly in our atmosphere.

Instead, scientists say the real threat may come from the asteroids we haven’t yet found, the ones large enough to wipe out a city, but small enough to slip past our telescopes unnoticed.

At a recent scientific conference, one of NASA’s top planetary defense experts acknowledged a sobering reality: Earth is not fully prepared, as quoted in a report by Newsweek.


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What keeps NASA’s planetary defense chief up at night?

Speaking at the American Association for the Advancement of Science (AAAS) conference in Arizona, NASA planetary defense officer Kelly Fast was candid about her biggest concern.

“What keeps me up at night is the asteroids we don’t know about,” she said, as per a report by The Sun.

Fast explained that smaller asteroids strike Earth frequently, but they typically don’t pose major threats. Large asteroids — the catastrophic, global-impact kind — are also less concerning because astronomers have already located and tracked them.

The real danger lies in the middle category: asteroids about 140 meters (roughly 459 feet) or larger. These so-called “city killer” asteroids could cause devastating regional damage if they were to hit Earth.

“They are most worried about ‘the ones in between, about 140 metres [around 459 feet] and larger, that could really do regional rather than global damage, and we don’t know where they are.’”

According to Fast, scientists estimate there are around 25,000 such near-Earth asteroids. So far, only about 40 percent have been identified.

"The ones in between, about 140 metres [around 459 feet] and larger, that could really do regional rather than global damage, and we don’t know where they are," she noted, as quoted in a report by Newsweek.

“It’s estimated there are about 25,000 of those, and we’re only about 40 percent of the way through,” Fast said. “It takes time to find them, even with the best telescopes.”

What are near-Earth objects and potentially hazardous asteroids?

Asteroids are rocky remnants from the early solar system, formed roughly 4.6 billion years ago. Most are clustered in the main asteroid belt between Mars and Jupiter.

However, some objects — known as near-Earth objects — have orbits that bring them into Earth’s orbital neighborhood, within 120 million miles of the sun.

NASA emphasizes that most near-Earth objects do not pose a threat. Their paths do not bring them especially close to Earth.

A smaller subset, called potentially hazardous asteroids (PHAs), demands closer monitoring. These are typically more than 460 feet wide and pass within 4.6 million miles of Earth’s orbit, as quoted in a report by Newsweek.

Still, officials stress that none of the currently known PHAs are expected to strike Earth anytime soon.

“The 'potentially hazardous' designation simply means over many centuries and millennia, the asteroid's orbit may evolve into one that has a chance of impacting Earth. We do not assess these long-term, many-century possibilities of impact,” said Paul Chodas, manager of the Center for Near Earth Object Studies (CNEOS), as quoted in a report by Newsweek.

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Did asteroid 2024 YR4 pose a real threat?

Concern briefly surged last year when data showed asteroid 2024 YR4 had a 3.1 to 3.2 percent chance of impacting Earth in 2032 — described at the time as “the highest impact probability NASA has ever recorded for an object of this size or larger.”

The figure was later revised. NASA announced that “the object poses no significant impact risk to Earth in 2032 and beyond.”

Even so, the episode served as a reminder of how quickly risk assessments can change — and how little time humanity may have to react if a real threat emerges.

The James Webb Space Telescope is currently tracking 2024 YR4. While Earth is no longer at risk, scientists estimate there remains about a 4 percent chance it could strike the moon, potentially producing a flash visible from Earth.


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Can NASA actually deflect an asteroid?

In 2022, NASA demonstrated that altering an asteroid’s path is technically possible. The agency’s DART (Double Asteroid Redirection Test) mission intentionally crashed a spacecraft into Dimorphos, a small moonlet orbiting a larger asteroid, at 14,000 mph. The impact successfully shifted the object’s trajectory.

But experts caution that a successful test does not equal a ready-to-launch defense system.

“We don’t have [another] Dart just lying around,” said Nancy Chabot, a planetary scientist at Johns Hopkins University who led the DART mission. “If something like YR4 had been headed towards the Earth, we would not have any way to go and deflect it actively right now.”

Scientists say Earth lacks an operational, on-demand asteroid defense capability. Building that readiness would require sustained investment and long-term planning, as per a report by The Washington Times.

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FAQs

How many dangerous asteroids remain undiscovered?
Scientists estimate about 25,000 near-Earth asteroids larger than 140 meters exist, and only around 40 percent have been found so far.

Can NASA currently deflect a threatening asteroid?
Although the DART mission proved deflection is possible, there is no ready-to-launch system available right now to actively stop an incoming asteroid.